1. Field of the Invention
The present invention relates to an optically functional device to control light by utilizing polarization-inverted structures in a LiTaO3 single crystal substrate, which is useful in various fields of e.g. optical information processing, optical processing technology, optical communication technology and optical measurement using a laser beam, a single crystal substrate for such an optically functional device, and a method of using such a single crystal substrate for the optically functional device.
2. Discussion of Background
A lithium tantalate (LiTaO3) single crystal (hereinafter sometimes referred to simply as xe2x80x9cLTxe2x80x9d) known as a typical ferroelectric single crystal, is used mainly as a substrate for a surface acoustic wave device. With respect to this crystal, a single crystal having a large diameter and a high compositional homogeneity can be supplied at a relatively low cost. Further, this crystal is transparent within a wide wavelength range of from ultraviolet to infrared, and it is possible to invert the ferroelectric polarization even at room temperature by applying a high electric field at a level of a few tens kV/mm. Accordingly, it has attracted attention in recent years also as a substrate for various optically functional devices such as non-linear optical devices or electro-optical devices, using the polarization-inverted structures.
Particularly, in recent years, it is expected to have second harmonic wave generating (SHG) devices of waveguide type developed to convert a semiconductor laser having a near infrared wavelength to a blue light having a half wavelength by a non-linear effect. Among them, a frequency conversion device is most thoroughly studied which employs an element of a structure having the polarization of an inorganic ferroelectric single crystal such as LT or a lithium niobate (LiNbO3) single crystal (hereinafter sometimes referred to simply as xe2x80x9cLNxe2x80x9d) periodically inverted, as a light source for high density recording/read out of optical disks. Such a frequency conversion device is based on a quasi phase matching (QPM) system, which is a system to carry out phase matching by compensating the difference between propagation constants of the fundamental wave and the higher harmonic wave with the periodic structures.
This system has many excellent characteristics such that high conversion efficiency can be obtained, collimation or diffraction limit condensation of the output light is easy, and useful materials or wavelengths are not particularly limited. As the periodic structures for QPM, structures having the sign of the SHG coefficient (the d33 coefficient) periodically inverted, are most effective to obtain high efficiency, and with a ferroelectric crystal, positive or negative of the d coefficient corresponds to the polarity of the ferroelectric polarization, whereby the technology to have the ferroelectric polarization domains periodically inverted, becomes important.
As a device employing this system, a green SHG device by means of an internal resonator using a Nd:YVO4 single crystal as the fundamental wave, has been reported, which employs a QPM element having periodically inverted structures formed by applying an electric field of about 21 kV/mm to a LT single crystal, as disclosed in a literature (Y. Kitaoka et al., Optics Letters, 21, p1974, 1996). Particularly, the LT single crystal has a large non-linear optical constant (d33 being 26 pm/V) comparable to the LN single crystal and is strong against optical damage as compared with the LN single crystal, and its fundamental absorption range extends to 280 nm, whereby it is expected to be useful as a frequency conversion material for short wavelengths.
Further, with respect to an optical device utilizing an electro-optical effect, for example, according to a literature (M. Yamada et al., Appl. Phys. Lett., 69, p3659, 1996), attention has been drawn to a cylindrical lens, a beam scanner, a switch, and an optical device to form polarization-inverted structures of a lens or prism shape in a LN single crystal as a ferroelectric crystal by applying a high electric field to the crystal and to polarize a laser beam passed therethrough by utilizing the electro-optical effect, as new optical devices. The LT single crystal which is transparent to a shorter wavelength than the LN single crystal, is considered to be promising as an excellent substrate material for an optical device using ultraviolet to visible light.
With respect to a frequency conversion device or an electro-optical device utilizing the polarization-inverted structures of a ferroelectric LT single crystal, so far reported, in each case, a commercially available LT single crystal having a congruent composition with no additive, has been employed as the substrate crystal, because the LT single crystal so far available has been limited to a crystal having a congruent composition grown by a Czochralski method which is capable of growing a crystal having a large diameter inexpensively, from the industrial viewpoint. With respect to the LT crystal, it is well known from the correlation diagram of temperature/compositional ratios (phase diagram) that the stoichiometric composition and the congruent composition do not agree with each other.
Only the congruent composition is a composition whereby the composition of the melt and the composition of the crystal will agree, and a crystal having a uniform composition can be grown over the entire crystal. Accordingly, the composition of the LT single crystal which is presently produced and used for various applications, is a congruent composition having a molar fraction of Li2O/(Ta2O5+Li2O) of about 0.483 (the molar ratio of Li/Ta being about 0.93). Accordingly, in the conventional LT single crystal having a congruent composition, the Ta component is excessive, whereby Ta ions as much as a few percent are substituted for Li ions (anti-site defects), and Li ion sites also have a few percent of vacancy defects. The influence of these defects may not be serious for an application to a surface acoustic wave device, but can not be neglected for an application to an optical device. Accordingly, it has been desired to develop a crystal having a composition close to the stoichiometric composition and having non-stoichiometric defects reduced, as a substrate to be used for optically functional devices.
As is apparent from the phase diagram, for example, in the case of a LT single crystal, a crystal having a composition close to the stoichiometric composition can be precipitated from a melt having a composition having a Li concentration higher than the stoichiometric composition. However, when it is attempted to grow a crystal having the stoichiometric composition by the Czochralski method which has been used as a means to produce a LT crystal having a large diameter in a large amount on an industrial scale, the excessive Li component will remain in a crucible along with the precipitation of the crystal, and the compositional ratio of Li/Ta in the melt will gradually change, whereby the compositional ratio in the melt will reach the eutectic point soon after the initiation of growing. Accordingly, the solidification ratio of the crystal is restricted to a level as low as about 10%, and the quality of the precipitated crystal has been so poor that it can not be used for an optically functional device.
The present inventors have previously invented a lithium tantalate single crystal close to the stoichiometric composition having a molar fraction of Li2O/(Ta2O5+Li2O) of from 0.495 to 0.50 (the molar ratio of Li/Ta being from about 0.98 to 1.00) and having the non-stoichiometric defect concentration of the congruent composition substantially reduced, as a novel substance different from the conventional commercially available LT crystal having a congruent composition, and have filed a Patent Application (JP-A-11-35393). Further, they have reported about this novel crystal in a literature as follows. Namely, as a means to develop a crystal of high quality by reducing such non-stoichiometric defects, the present inventors have proposed a method of growing a crystal while continuously supplying the material (hereinafter referred to simply as the continuous supply method), for example, in a literature (Y. Furukawa et al. J. Crystal Growth 197, p889, 1999).
Specifically, it is a method wherein the molar fraction of Li2O/(Ta2O5+Li2O) in the melt for growing is adjusted to be from 58.0 to 59.0 (the molar ratio of Li/Ta being from about 1.38 to 1.44) i.e. the Li component being excessive, and the crucible is made to have a double structure so that from the inner crucible, a LT crystal close to the stoichiometric composition is pulled up and the weight of the crystal being pulled up is measured at all times to obtain the growth rate, and a starting material powder having the same stoichiometric composition as the crystal, is continuously supplied between the outer and inner crucibles at that rate. By employing this method, it becomes possible to grow a long crystal, and a crystal solidification ratio of 100% based on the amount of the material supplied, has been realized. It is reported that the crystal grown by this method has a Curie temperature of from 675 to 685xc2x0 C., which is far higher than the Curie temperature of 601xc2x0 C. of the conventional crystal having a congruent composition, and a lithium tantalate single crystal close to the stoichiometric composition with excessive Ta, has been obtained.
Further, recently, the present inventors have reported that with the above-mentioned crystal close to the stoichiometric composition with excessive Ta, the applied voltage required for the polarization inversion is at a level of {fraction (1/10)} of the voltage heretofore applied (K. Kitamura et al., Appl. Phys. Lett., 73, p3073, 1998). Namely, they have reported that the presence of a few percent of non-stoichiometric defects (anti-site defects or vacancy defects) in the conventional crystal having a congruent composition, possibly increases the applied voltage required to form the periodic polarization structures or the optical characteristics which the LT crystal essentially has.
Further, it is known that with LT single crystals having a congruent composition, the optical damage resistance varies as much as a few digits among the crystals. However, as compared with the conventional congruent composition, the lithium tantalate single crystal close to the stoichiometric composition with excessive Ta, has an improved optical damage resistance against irradiation with a green optical laser having a wavelength of 532 nm, and the fluctuation among the crystals tends to be small to some extent, as reported (Yasunori Furukawa et al., Collection of papers delivered at the 60th Applied Physics Society Lecture Meeting 2b-ZB-1, the third volume, paragraph 1001, 1999).
Further, it is known that a LT single crystal close to the stoichiometric composition having MgO added, shows a optical damage resistance superior to the conventional congruent composition against irradiation with a green color optical laser having a wavelength of 532 nm (Akio Miyamoto et al., Collection of papers delivered at the 4th Artificial Crystal Discussion Meeting 27A, paragraph 75, 1999). It is also known that with any composition, the optical damage of a LT single crystal with excessive Ta is more likely to result as the wavelength of the irradiated laser becomes shorter, and the optical damage resistance in the vicinity of a wavelength of 400 nm is lower by at least two digits than the optical damage resistance at a wavelength of 532 nm. In such a case, Mg is substituted also at Li sites, and as the amount of Mg added, increases, the Li/Ta molar ratio becomes small as compared with the crystal having no additive, and the Li/Ta molar ratio of the obtained crystal is from 0.95 to 1.0.
A research on a near infrared region bulk OPO device as a quasi-phase-matching (QPM) device employing a LT single crystal (Curie temperature: 680-685xc2x0 C.) close to the stoichiometric composition of a lithium tantalate single crystal with excessive Ta, has been reported, for example, in a literature (Takaaki Hatanaka et al., Preparatory papers for the 60th Applied Physics Society Lecture Meeting 2a-k-7, third volume, p.932, 1999). Namely, a periodic electrode is provided on one side of a LT single crystal close to the stoichiometric composition of z-cut, and a uniform electrode is provided on the other side, and a pulse voltage of about a few kV/mm is applied through these electrodes, whereby a near infrared region bulk OPO device having a thickness of from 1 to 2 mm is prepared relatively easily. However, it is difficult to carry out the polarization inversion uniformly, and preparation of such a device is limited to formation of polarization-inverted structures in a very small area, and it has not been possible to form polarization inversion over a large area.
Further, according to a literature (Koichiro Nakamura et al., Preparatory papers for the 47th Applied Physics Society Lecture Meeting 30p-ZD-3, third volume, p.1105, 2000), preparation of an OPO device having a crystal substrate thickness of 3 mm has been studied by using as a substrate, a LT single crystal close to the stoichiometric composition with excessive Ta which has been previously invented by the present inventors, but control of polarization inversion has been more difficult, whereby a bulk OPO device employing it as the substrate, has not been obtained.
In order to form polarization-inverted structures on a substrate made of a ferroelectric single crystal and to realize an optically functional device utilizing the interaction between the electro-optical effect and the non-linear optical effect of light passed through the polarization-inverted portions, the most important technique is to prepare from a few to as many as several hundreds polarization-inverted structures of a few xcexcm to several tens xcexcm sizes uniformly and with high precision.
As a method for forming polarization inversion, an electron beam irradiation method or a voltage application method is well known and commonly used. Such an optically functional device is used by passing light through the polarization-inverted portions, and if there is an optical distortion or loss at the respective polarization-inverted boundaries, very substantial optical non-uniformity will result as the entire device, whereby a device with high efficiency can not be realized.
An optical distortion will be formed at the boundary of a polarization-inverted portion, whereby a change in the refractive index as large as 10xe2x88x923 to 10xe2x88x924 or larger will result. A serious problem has been pointed out in a literature (V. Gopalan et al., J. Appl. Phys., vol. 80, p.6104, 1996) such that the change in the refractive index brings about scattering of the laser beam passed therethrough, whereby the operation of the device will depart from the ideal condition, and the device efficiency will decrease. It is reported that in order to remove the optical distortion caused by such a large change in the refractive index, it is necessary to heat the LT single crystal having the polarization-inverted structures formed, at a temperature of 350xc2x0 C. for about 12 hours to relax the optical distortion.
Further, in the above-mentioned literature, (Y. Kitaoka et al., Optics Letters, 21, p1972, 1996), it is reported that heat treatment is necessary after formation of polarization inversion by application of an electric field, also for a green color optical frequency conversion device by means of an internal resonator using a Nd:YVO4 single crystal as the fundamental wave, employing a QPM device having periodically inverted structures formed by applying an electric field of about 21 kV/mm to a LT single crystal as a ferroelectric crystal. It is reported that in this case, the propagation loss within the crystal can be reduced to a level of from 2.5% to 0.1%, by heating the crystal at a temperature of at least 100xc2x0 C.
In a conventional voltage application method, it is common that a LT single crystal having a congruent composition of z-cut is employed, and a periodic electrode is provided on one side of the crystal, and a uniform electrode is provided on the other side, and the sample is maintained at room temperature or heated to a level of about 200xc2x0 C., and a pulse voltage is applied through the electrodes to invert the polarization of the portion immediately beneath the periodic electrode towards the z-axis direction. In the case of a conventional LT single crystal having a congruent composition, an applied voltage as high as at least 21 kV/mm is required for the polarization inversion.
Such a polarization inversion technique in the LT and LN single crystals, is to forcibly change the polarization i.e. the positions of Ta and Nb or Li ions in the crystal, at a temperature of not higher than the Curie temperature. With respect to the LT and LN single crystals, it is suggested that the high voltage required for the polarization inversion may not necessarily be the direct cause for an optical distortion.
Namely, in the above-mentioned literature (A. Harada et al., Optics Letters, 22, p805, 1997), it is reported that with a LN single crystal having a congruent composition having 5 mol % of MgO added, the voltage required for the polarization inversion can be reduced to a level of about ⅕ of the voltage required for a usual congruent composition, but, even if this material is employed, heating at a temperature of 500xc2x0 C. for three hours is required to remove an optical distortion in a case where a SHG laser having polarization-inverted structures formed in a periodic cycle of 4.75 xcexcm in a LN single crystal having MgO added, by means of a corona discharge method, is to be prepared.
When the polarization-inverted boundaries of such a device having polarization-inverted structures formed in a substrate made of a conventional LT crystal having a congruent composition, were observed by a polarization microscope, it was observed, as shown in FIG. 1(a), that the boundaries of the polarization-inverted portions were not smooth, and large optical distortions were observed at all of the polarization-inverted boundaries. Further, when the working laser beam was passed across the polarization-inverted portions, a very large propagation loss at a level of a few percent to a few tens percent was observed. Formation of such optical distortions at the polarization-inverted boundaries, not only creates a problem of a large propagation loss but also makes it necessary to provide an extra heat treatment step for the preparation of an optically functional device to relax the optical distortions.
Further, another serious problem is that a pyroelectric effect is likely to form at polarization-inverted portions of a few xcexcm size once formed by e.g. an electric field application method at portions of a single polarization substrate during the heat treatment to remove distortions, whereby the crystal is likely to be broken, or the sizes or positions of the polarization-inverted portions are likely to change although very slightly. Such a change creates a serious problem for the preparation of a device with high efficiency and good reproducibility.
The present inventors have continuously studied the characteristics of the LT single crystals as ferroelectric single crystals in order to solve the above-mentioned problems of the prior art and have found that with a LT single crystal having a composition close to the stoichiometric composition, even when polarization inversion is formed, optical distortions or propagation losses of light at the polarization-inverted boundaries are very small, and by using this single crystal as the substrate, it is possible to prepare an optically functional device having polarization-inverted structures, which has excellent properties.
Namely, the present invention provides an optically functional device comprising a ferroelectric single crystal substrate and polarization-inverted structures formed at portions of the substrate at a temperature of not higher than the Curie temperature by an electron beam scanning irradiation method or a voltage application method and designed to control light passed through the polarization-inverted portions, wherein a LiTaO3 crystal having a molar ratio of Li/Ta within a range of from 0.95 to 1.02, is used as the substrate, so that the propagation loss of light passed through the polarization-inverted portions immediately after formation of the polarization-inverted structures, is not more than 2%.
Further, the present invention provides an optically functional device comprising a ferroelectric single crystal substrate and polarization-inverted structures formed at portions of the substrate at a temperature of not higher than the Curie temperature by an electron beam scanning irradiation method or a voltage application method and designed to control light passed through the polarization-inverted portions, wherein a LiTaO3 crystal having a molar ratio of Li/Ta within a range of from 0.95 to 1.02, is used as the substrate, so that the change in the refractive index of the polarization-inverted boundaries is not more than 1xc3x9710xe2x88x924 without a heating step to remove an optical distortion at the polarization-inverted boundaries due to the directional inversion of spontaneous polarization in the ferroelectric crystal.
Further, the ferroelectric single crystal substrate to be used in the above optically functional devices is made of a LiTaO3 crystal having a molar ratio of Li/Ta within a range of from 0.95 to 1.00 and containing from 0.1 to 4.8 mol % of at least one element selected from the group consisting of Mg, Zn, Sc and In, as doped.
Still further, the present invention provides a single crystal substrate for an optically functional device to control light passed through a polarization-inverted portions, which is a ferroelectric single crystal substrate having polarization-inverted structures formed at a temperature of not higher than the Curie temperature by an electron beam scanning irradiation method or a voltage application method, wherein a LiTaO3 crystal having a molar ratio of Li/Ta within a range of from 0.95 to 1.02, is used as the substrate, so that a propagation loss of light of not more than 2% and an optical distortion of not more than 1xc3x9710xe2x88x924 are obtainable without a heat treatment.
The present invention further provides a method for improving the frequency conversion efficiency of an optically functional device, wherein a LiTaO3 crystal having a molar ratio of Li/Ta within a range of from 0.95 to 1.02, is used as a substrate for the optically functional device to carry out the frequency conversion of a laser entered into a single crystal having periodically inverted polarization structures by means of a nonlinear optical effect.
Furthermore, the present invention provides a method for improving the driving efficiency of an optically functional device, wherein a LiTaO3 crystal having a molar ratio of Li/Ta within a range of from 0.95 to 1.02, is used as a substrate for the optically functional device to control polarization or condensing of a laser beam entered into a single crystal having polarization structures inverted in a prism or lens shape by means of an electro-optical effect.